In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
Conventionally, imaging of the imageable element with ultraviolet and/or visible radiation is carried out through a mask, which has clear and opaque regions. However, direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Imageable elements have been developed for use with infrared lasers. These imageable elements typically require processing in a developer to convert them to lithographic printing plates. Developers are typically aqueous alkaline solutions, which may also contain substantial amounts of organic solvents. Because of their high pH and the presence of organic solvents, disposal of substantial quantities of developer is expensive and can cause environmental problems. Processing of the imaged imageable element in a developer also introduces additional costs in, for example, the cost of the developer, the cost of the processing equipment, and the cost of operating the process.
To overcome these disadvantages, imageable elements that do not require processing in a developer have been developed. One approach is the use of elements in which the imageable layer comprises a “switchable polymer.” During thermal imaging, these polymers typically undergo a chemical reaction in which highly polar moieties are either created or destroyed so that the surface of the imageable layer is changed from oleophilic to hydrophilic or from hydrophilic to oleophilic. No only do these imageable elements not require processing in a developer, they can be imaged on-press, which eliminates the step of mounting the element in a separate imaging device.
Leon, U.S. Pat. No. 6,447,978, the disclosure of which is incorporated herein by reference, discloses an imageable element comprising a support having a hydrophilic imageable layer. The imageable layer comprises a hydrophilic heat-sensitive polymer that contains quaternary ammonium carboxylate groups. When the imageable layer is thermally imaged, the imaged regions are rendered more oleophilic than the unimaged regions. However, the resulting printing plates show start up blinding on press. That is, it can take several hundred impressions before good quality prints of good, uniform density are produced. Thus a need exists for a method for forming an image using a no-process imageable element useful as a printing plate precursor that does not have this problem.